Seznam abstraktů 2023: SEKCE B - TVÁŘENÍ KOVŮ

All demanding modern applications such as structural, energy and automotive, require steels with an increasingly high level of strength, toughness and formability. Especially those that utilise low to medium C chemistries where impact toughness and weldability are of paramount importance.
In order to achieve this then a fine-grained microstructure with a homogeneous through thickness grain size distribution is highly desirable combined with the potential for forming finely dispersed and well distributed precipitated strengthening particles.
CBMM has developed together with industrial experts, various academics and research institutes, numerous tools and methodologies that can help the modern steel manufacturer in producing a particular product to fulfil the customer and standard specifications in the most cost effective and sustainable manner. The focus is on chemical alloy design and processing in order to achieve desired properties where all important stages in the process are evaluated from melting, refining and continuous casting through to reheating, hot rolling and cooling conditions. The work to be presented here is mainly centred around the austenite conditioning during hot rolling where the objective is to attain a homogeneous microstructure suitable for improved ductile properties such as toughness, global and local formability. Dependent upon the required strengthening mechanism and subsequent processing, then a well optimised fine grain size microstructure may also be desired. The calculated outputs include the average grain size and distribution post reheat, post hot-rolling and final transformed microstructure.

During recrystallization annealing after cold forming, it has been repeatedly observed that there is a critical range of strains beyond which enormous coarsening of the statically recrystallized grain occurs. It can be assumed that a similar behaviour can also be expected during hot rolling with static recrystallization (SRX) between passes, especially in the first passes at the highest temperatures. The aim of this paper is to experimentally determine the effect of strain during on the grain size and to find the critical strain range for grain coarsening. For this purpose, a plain strain compression test (PSCT) was performed on the Hydrawedge II simulation module cooperating with the electrical and hydraulic systems of the HDS-20 simulator (Gleeble 3800). A total of 18 tests were performed for three different deformation temperatures (1000, 1050 and 1100 °C) and 5 different interpass times after deformation (1 to 200 s). Thanks to PC simulation using FEM in the Simufact Forming program, the equivalent strain distribution in the deformed sample was determined. Thanks to this, it was possible to determine the dependence of the size of the original austenite grain after deformation (obtained metallographically) on delay at a given temperature for strain range of 0.02 to 0.51. Based on the experiments, graphs of the dependence of the size of the original austenite grain on the equivalent strain were compiled. The results showed that in the strain range of 0.05 to 0.15, there was an enormous grain coarsening, especially at the highest temperatures. The achieved results were compared with theoretical values obtained by means of calculations of SRX kinetics, recrystallized grain size and grain coarsening kinetics.

Hot forming processes are influenced by numerous thermomechanical-metallurgical material phenomena occur which interact strongly. In particular, the strains of transformation-induced plasticity (TRIP) occurring during the forming process have a considerable influence on the deformations and residual stresses of the components. As TRIP strains are anisotropic strains they depend on the orientation and magnitude of the stress states superimposed to the phase transformation during cooling. By numerical modelling the impact of TRIP effect can be analysed and taken into account during process design. However, required material data are poorly accessible to non-existent in the literature. Therefore, this work focuses on the determination of characteristic values of TRIP for the materials AISI 4140 and AISI 52100 considering dynamic effects. Tests were performed using hollow specimens which were thermo-mechanically loaded in this work. An external stress was applied shortly before the start of the transformation and, in the case of reversible transformation plasticity, selectively released during the transformation phase. In this way, it was possible to determine phase-specific as well as load-dependent reversible transformation plasticity effects. The determined values for TRIP effect with and without backstress were transferred to a FE simulation and successfully validated with an experimental comparison. The material models and subroutines created are now to be validated on the basis of experimental forging tests by comparing distortion and residual stresses.

This study deals with the unconventional processing of Cu-Al2O3 powder material using direct consolidation. Direct consolidation is the process of preparing powder materials without including sintering in the process. The first part is dedicated to determining the material constants of the Haensel-Spittel equation model for the Cu-Al2O3 material. In the second part, we deal with the use of the obtained model to predict material behavior using FEM simulation of the cold rotary swaging process. This study is complemented by a metallographic examination of the material.

By cold forming metastable austenitic steels below 0 °C, the strength is increased through martensite phase transformation in addition to strain hardening. This deformation-induced effect enables the use of corrosion-resistant materials for highly loaded applications such as bearings under oxidative atmospheres, where the use of conventional corrosion-prone steels is unsuitable. In this context, components made of austenitic steel could enable an economic application through a sustainable manufacturing process. The aim of this study is the modification of the functional surface layer of angular ball bearing rings made of X5CrNi18-10 (AISI 304) through bulk forming to increase their service life.
Forming parameters and tool settings are determined numerically to obtain the material flow and prevent tool cracking, since tool steels cannot withstand high mechanical loads at low process temperatures. After forming, local martensite formation in the surface layer of the angular ball bearing rings is detected through magnetic induction and metallographic analyses. The resulting increase in residual stress is demonstrated through X-ray diffraction and correlated to the achieved surface hardness. The service life of the modified inner angular ball bearing rings is examined on a developed bearing test stand and compared with those of conventionally machined bearings.

Hybrid components are made by combining two or more different materials and used in applications where monolithic components are not suitable due to their limited functionality or performance. Combining steel and aluminum in a technical component for power transmission provides the advantages of locally improved strength and reduced weight. However, the joining zone is critical for the component’s performance because of brittle intermetallic phases which may develop, which is why suitable measures must be taken to achieve a sufficient bond strength. The further forming of pre-joined hybrid semi-finished products allows the joining zone to be positively influenced, both geometrically and mechanically. In this study, components consisting of steel (20MnCr5) and aluminium (EN AW-6082) were first joined by friction welding and formed using four different impact extrusion processes to increase the joint interface area in the compound. In order to evaluate the influence on the bond quality, SEM images, impact bending tests and tensile tests were carried out. Depending on the adjusted interfacial area enlargements, an influence on the formation of the intermetallic phase and on the mechanical bond strengths could be observed. In contrast to specimens with an unaffected joining zone geometry after forming, those with a developed spherical joining zone and thus an enlarged interface (up to 80%) achieved higher bond strengths (up to 26%) and were characterized by local joint surfaces without a brittle intermetallic phase.

Copper is a very popular electro-conductive material, however, the mechanical properties of pure Cu are low. They can be typically improved by (micro)alloying, or via structure modifications introduced by optimized deformation and thermomechanical treatments. Designing a Cu-based composite, possibly strengthened by dispersions of fine oxides, is another way how to favourably improve the strength properties of Cu. In this study, we performed mechanical alloying of a Cu powder with a powder of Y2O3 oxide, which is known to have highly strengthening effects on metallic materials. After mixing, we sealed the powder mixture into evacuated tubular Cu containers (i.e. cans). As for the consolidation procedure, we applied direct consolidation of the canned powders via the intensive plastic deformation method of rotary swaging, performed at warm and cold conditions. Subsequently, we subjected the swaged conductors to measurements of electric conductivity and detailed structure observations. The results revealed that the applied swaging ratio was sufficient to fully consolidate the canned powders as the final conductor was unrecognizable from a cast alloy from the viewpoints of visual and structure assessment. In other words, the structure did not exhibit any voids or remnants of powder particles. The observed fine grains with homogeneous dispersion Y2O3 oxide particles provided substantial improvement of the mechanical properties, as proven by microhardness measurements. Moreover, the electric properties remained highly favourable.

The AISI 316L stainless steel is one of the most used 3D printed powder materials due to its good properties. It goes without saying that the properties of the 3D printed steel are naturally less or more different compared to the material which was prepared conventionally. One of the important material characteristics is its flow stress evolution under various thermomechanical circumstances, which has a direct connection to the deformation behavior. The hot deformation behavior of the examined 3D printed steel is in the frame of the submitted research studied with regard to the peak stress levels experimentally achieved under a wide temperature and strain rate range, as well as from the point of view of the onset of a dynamic softening phenomenon - in addition with an emphasis on the corresponding mathematical description. The obtained results indicate a minimal and maximal flow stress level of 65 MPa and 381 MPa as for the combination of 1523 K - 0.1 s−1 and 1173 K - 100 s−1, respectively. The experimentally achieved peak point coordinates have been described with a high accuracy - corresponding to the Pearson’s correlation coefficient of 0.8238 and 0.9919 as regards to the peak strain and peak stress, respectively.

One of the most important parameters used for qualitative evaluation of lubricants in cold drawing of steel wires is the coefficient of friction. Its magnitude cannot be determined directly, but it can be determined from the measured total drawing force (TDF) or total drawing torque (TDT). In addition to the coefficient of friction, the TDF is mainly influenced by the size of the partial strain during pass and also by the geometry of the die, which is mainly described by the angle of the deformation (conical) part of the die. In order to determine the coefficient of friction for the individual analysed lubricating oils, a series of experiments was carried out, consisting of drawing a phosphated low-carbon steel wire with a diameter of 5.5 mm in one pass on a Koch KGT direct drawing machine. In total, two single pass strain values were used for the evaluation of four oils (when drawn to a diameter of 5.1 resp. 4.9 mm, it was 14 resp. 20.6 %) and 6 different angles of the deformation part of the die (8 to 18°). The TDF was determined from the measured TDT and the coefficient of friction was determined inversely from the Siebel TDF equation. For a strain of 20.6 %, the correlation between the measured and calculated values as a function of angle was high, but at a strain of 14 % the data showed a low correlation. Therefore, the data were corrected using the contact pressure value, which is also a function of die geometry. Thanks to this modification, a new equation was established for calculating the frictional component of the TDF under conditions of mixed friction.